It is well known that for boilers and heaters, high excess air results in large thermal efficiency losses, and the excess air should be monitored and maintained at specific levels as required to combust the fuel. Traditional excess air levels for fossil fuels are:    Oil 3% (˜1% O2 by volume)    Natural Gas 5% (˜2% O2 by volume)    Coal 20% (˜3% O2 by volume)
These excess air levels could vary slightly depending upon the application. However, there are large numbers of industrial heaters and furnaces that require significantly higher excess air levels which are well beyond what is needed to combust the fuel properly. These levels have conventionally been considered as acceptable and normal within the context of certain processes.
There are numerous reasons for high excess air, including the following.
Firstly, and most commonly, the high excess air may be required to maintain the heat transfer rate of the process. Many applications require high convective rates to transfer heat from the flame and flue gas into a product or heat load, and the proportion of heat transfer which is convective or radiant will vary depending upon the furnace layout and operation.
Secondly, excess air can be used to moderate flame and furnace temperatures. For many applications, typical flame temperatures at close to stoichiometric ratios tend to be around 3600° F., which exceeds the maximum refractory brick operating temperatures—typically 2800° F. or significantly less—so that excess air may be used to maintain the integrity of refractories and other structural elements of the furnace.
Thirdly, in drying and curing applications, the humidity level in the furnace must be controlled, which is conventionally done by increasing the excess air, and thereby lowering the partial pressure of moisture within the process.
Fourthly, excess air can be used to control the levels of flammable vapours which may be released in flue streams, by diluting these vapors well below the lower flammability limit.
The types of combustion applications which typically use high excess air include the following:    Spray and solids drying    Curing    Induration of ore pellets    Annealing    Forging    Heating
Large amounts of excess air generally result in very large losses within the process. For example, it has been found that the flue gas in iron ore furnaces may contain oxygen at approximately 19.0% to 19.5% by volume, which corresponds to approximately 1400% to 2100% excess air. The dry gas loss increases exponentially as the O2 in the flue gas approaches the value for the oxygen in air, which is approximately 21% by volume.
In applications such as induration furnaces, very high excess air is required in order to meet the need for convective heat transfer in the various sections of the furnace. In such furnaces, the key heat transfer zone in the furnace is the combustion zone, within which are three primary modes of heat transfer to the product, that is, convection, radiation and conduction. Radiation consists of both direct luminous radiation from the flame envelope as well as cavity radiation within the physical geometry of the zone. Radiation is a strong function of both flame and mean cavity temperature, whereas convection is a strong function of both flue gas velocity and the temperature of the flue gas passing through the pellet bed. The flame temperature will increase if less combustion air is used; however, the convection drops off dramatically. The convection rate must be maintained throughout the bed. The third mode of heat transfer is conduction within the pellet bed.
Within the other zones of the furnace convective heat transfer predominates; however, it is both a function of flue gas velocity through the bed and the temperature of the flue gas. By increasing temperature in these zones slightly it may be possible to maintain a similar heat transfer characteristic within the specific zones while decreasing velocity.
Clearly, any reduction in the amount of excess air used will result in increased efficiency and decreased fuel consumption, with consequent economic benefits. Further, a decrease in excess air will also decrease the amount of flue gas requiring treatment.
From prior art it is known that there are various methods of recirculation of flue gases in combustion systems which do not use high excess air. However, such methods are directed at controlling the flame temperature, NOx, and steam temperatures and are not intended to improve efficiency of high excess air systems.
It has now been found that for high excess air furnaces, such as induration furnaces, controlled and selective recirculation of exhaust gases from the drying, pre-heating, and combustion zones can substantially reduce the amount of excess air required by the system, while maintaining the required convective heat transfer.
The invention therefore seeks to provide a flue gas recirculation system for combustion systems which use high levels of excess air, such as drying, curing, induration and other systems, including but not limited to those noted above. In its most general conception the system uses the heat exhausted in the flue gas and re-introduces it into the process to replace heat input from fuel. The various methods for re-introduction can vary from one process to another. The invention is particularly advantageous for use for induration furnaces. However, although particular reference is made in the discussion below to the specific features and requirements for such furnaces, the features of the invention are equally applicable to such other excess air systems in general.
The invention therefore seeks to provide a flue gas recirculation system for a combustion system, the combustion system comprising in sequence at least one pre-combustion drying zone, at least one combustion zone, and at least a first cooling zone, the recirculation system comprising
(i) a plurality of exhaust gas outlets comprising at least one exhaust gas outlet provided respectively to each pre-combustion drying zone and each combustion zone, and constructed and arranged to remove a gaseous flow from each said zone;
(ii) at least one cooling zone intake means provided to each cooling zone;
(iii) at least one flue gas delivery means each having at least one recirculation intake means and at least one delivery outlet, at least one of the plurality of exhaust gas outlets being operatively connectable to one of the recirculation intake means to selectively deliver at least part of the respective gaseous flow as a recirculation flow to the flue gas delivery means, and each delivery outlet being selectively operatively connectable to a selected one of the cooling zone intake means; and
(iv) control means operatively connected to the flue gas delivery means to selectively control and direct the recirculation flow.
In one aspect of this embodiment, the at least one exhaust gas outlet operatively connectable to one of the recirculation intake means comprises a selected one of the at least one exhaust gas outlet provided to the least one combustion zone.
Preferably, the at least one pre-combustion drying zone comprises an updraft drying zone and a downdraft drying zone, and in a further aspect of the invention, the at least one exhaust gas outlet operatively connectable to one of the recirculation intake means comprises a selected one of the at least one exhaust gas outlet provided to the least one combustion zone and a selected one of the at least one exhaust gas outlet provided to the updraft drying zone.
Preferably, the combustion system further comprises at least one pre-heating zone.
Preferably also the at least one cooling zone comprises a first cooling zone and a second cooling zone, and the flue gas delivery means is constructed and arranged to deliver the recirculation flow to the cooling zone intake means of the first cooling zone.
Preferably, the combustion system is for an operational use selected from at least one of curing, drying, induration, heating, annealing and forging.
The invention further seeks to provide a method of recirculation of flue gas for a combustion system, the combustion system comprising in sequence at least one pre-combustion drying zone, at least one combustion zone, and at least a first cooling zone, the method comprising the steps of
(i) providing a plurality of exhaust gas outlets comprising at least one exhaust gas outlet provided respectively to each pre-combustion drying zone and each combustion zone to allow a gaseous flow selectively to and through each exhaust gas outlet;
(ii) providing at least one cooling zone intake means to each cooling zone;
(iii) providing at least one flue gas delivery means each having at least one recirculation intake means and at least one delivery outlet;
(iv) selecting at least one exhaust gas outlet from the plurality of exhaust gas outlets, and connecting the selected exhaust gas outlet to one of the recirculation intake means;
(v) selectively delivering at least part of the gaseous flow from each selected exhaust gas outlet as a recirculation flow to the flue gas delivery means;
(vi) selectively delivering at least part of the recirculation flow through one of the delivery outlets to the selected cooling zone intake means;
(vii) monitoring at least periodically the temperatures and pressures in each cooling zone receiving the recirculation flow to determine temperature and pressure values; and
(iv) adjusting the recirculation flow in response to the determined temperature and pressure values.
In one aspect of this embodiment, step (iv) comprises selecting an exhaust gas outlet from the combustion zone.
In another aspect of this embodiment, the at least one pre-combustion drying zone comprises an updraft drying zone, and step (iv) further comprises selecting an exhaust gas outlet from the updraft drying zone.
The recirculation systems of the invention are compatible with the conventional configurations of many or most high excess air systems, and are particularly advantageous for systems wherein the at least one pre-combustion drying zone comprises an updraft drying zone and a downdraft drying zone; and/or wherein the combustion system further comprises at least one pre-heating zone and/or multiple cooling zones, such that the flue gas delivery means is preferably constructed and arranged to deliver the recirculation flow to the cooling zone intake means of the first of the cooling zones.
As noted above, the recirculation systems of the invention can be used for a wide range of operational end uses, such as at least one of curing, drying, induration, heating, annealing and forging.